Effects
of diesel soot composition and accumulated vehicle mileage
on soot oxidation characteristics were examined. Four soot samples
were extracted from the crankcase oils of diesel engines that had
accumulated different mileages. Carbon black was used as a comparative
example. Soot structure was studied in situ using X-ray diffraction
as it was oxidized to temperatures as high as 700 °C. The soot
from older engines exhibited a higher increase in lattice spacing
(d
002) with an increase in temperature
that resulted in soot samples being at lower temperatures, thereby
reducing the oxidation resistance. Composition of the residues after
oxidation was studied using X-ray diffraction, energy-dispersive spectroscopy,
and X-ray absorption near-edge structure. Oxidation residue of the
soot samples is made up of decomposed lubricant additives compounds
and debris from wear and tribofilms. XRD phase analysis showed that
crystalline compounds in soot are CaSO4, CaZn2(PO4)2, Ca3(PO4)2, Zn3(PO4)2,
and ZnO. The turbostratic structure of all the soots irrespective
of engine age is similar prior to oxidation; however, the embedded
crystalline and amorphous species in the soot change with accumulated
mileage. Surface area of the soot measured using BET was found to
be inversely proportional to the weight of residue.